DEPARTMENT OF COMPUTER SCIENCE AND ENGINEERING

COMPUTER NETWORKS

PART A

UNIT – I

1. What is mean by data communication?

Data communication is the exchange of data (in the form of 1s and 0s) between two
devices via some form of transmission medium (such as a wire cable).

2. What are the three criteria necessary for an effective and efficient network?

The most important criteria are performance, reliability and security.

Performance of the network depends on number of users, type of transmission
medium, the capabilities of the connected h/w and the efficiency of the s/w.
Reliability is measured by frequency of failure, the time it takes a link to recover from
the failure and the network’s robustness in a catastrophe.
Security issues include protecting data from unauthorized access and viruses.

3. What are the three fundamental characteristics determine the effectiveness

of the data communication system?

The effectiveness of the data communication system depends on three fundamental

characteristics:
Delivery: The system must deliver data to the correct destination.
Accuracy: The system must deliver data accurately.
Timeliness: The system must deliver data in a timely manner.

4. What are the advantages of distributed processing?

Advantages of distributed processing include security/encapsulation, distributed

databases, faster problem solving, security through redundancy and collaborative
processing.

5. Why are protocols needed?

In networks, communication occurs between the entities in different systems. Two

entities cannot just send bit streams to each other and expect to be understood. For
communication, the entities must agree on a protocol. A protocol is a set of rules that
govern data communication.

6. Why are standards needed?

Co-ordination across the nodes of a network is necessary for an efficient communication.
If there are no standards, difficulties arise. A standard provides a model or basis for
development to which everyone has agreed.

7. For n devices in a network, what is the number of cable links required for a
mesh and ring topology?

Mesh topology – n (n-1)/2

Ring topology – n

8. What is the difference between a passive and an active hub?

An active hub contains a repeater that regenerates the received bit patterns before sending
them out. A passive hub provides a simple physical connection between the attached
devices.

9. Assume 6 devices are arranged in a mesh topology. How many cables are
needed? How many ports are needed for each device?

Number of cables=n (n-1)/2=6(6-1)/2=15

Number of ports per device=n-1=6-1=5

10. Group the OSI layers by function.

The seven layers of the OSI model belonging to three subgroups. Physical, data link and
network layers are the network support layers; they deal with the physical aspects of
moving data from one device to another. Session, presentation and application layers are
the user support layers; they allow interoperability among unrelated software systems.
The transport layer ensures end-to-end reliable data transmission.

11. What are header and trailers and how do they get added and removed?

Each layer in the sending machine adds its own information to the message it receives
from the layer just above it and passes the whole package to the layer just below it. This
information is added in the form of headers or trailers. Headers are added to the message
at the layers 6,5,4,3, and 2. A trailer is added at layer2. At the receiving machine, the
headers or trailers attached to the data unit at the corresponding sending layers are
removed, and actions appropriate to that layer are taken.

12. The transport layer creates a communication between the source and
destination. What are the three events involved in a connection?

Creating a connection involves three steps: connection establishment, data transfer and
connection release.

13. What is CSMA/CD?

CSMA/CD stands for Carrier Sense Multiple Access with Collision Detect. Ethernet is a
multiple access network (shared link) and the carrier sense means that all the nodes can
distinguish between an idle and busy link. Collision detect means that a node listens as it
transmits and can therefore detect when a frame it is transmitting has interfered with a
frame transmitted by another node.

14. What is transceiver?

Transceiver is a small device directly attached to the tap and detects when the line is idle
and drives the signal when the host is transmitting.

15. What is Repeater?

Repeater propagate the signal from one segment to another

16. Write short notes on Ethernet address?

Ethernet host in the world has a unique address. The address belongs to the adaptor, not
the host; it is usually burned into ROM. it has six hexadecimal numbers separated by
colons. Example 8:0:2b:e4:b1:2.

17. Why Ethernet is called 1 persistent protocol?

Ethernet is said to be a 1 persistent protocol because an adaptor with a frame tosend

transmits with probability 1 whenever a busy line goes idle.

18. What is an exponential back off?

Once an adaptor has detected a collision and stopped its transmission, it waits a certain
amount of time and tries again. Each time it tries to transmit but fails, the adaptor doubles
the amount of time it waits before trying again. This strategy of doubling the delay
interval between each transmission attempt is a general technique known as exponential
back off.

19. What are the advantages of Ethernet?

Easy to admin and maintain

No switch, Router and configuration table
Easy to add a new Host
Inexpensive one

20. Write short notes on Token Ring?

A number of stations connected by transmission links in a ring topology.
Information flows in one direction along the ring from source to destination and back to
source. Medium access control is provided by a small frame, the token, which circulates
around the ring when all stations are idle. Only the station possessing the token is allowed
to transmit at any given time.

21. Write short notes on FDDI?

• FDDI uses a ring topology of multimode or single mode optical fiber transmission links
operating at 100 Mbps to span up to 200 kms and permits up to 500 stations.
• Employs dual counter-rotating rings.
• 16 and 48-bit addresses are allowed.
• In FDDI, token is absorbed by station and released as soon as it completes the frame
transmission {multi-token operation}.

22. What are the 2 different classes of traffic used in FDDI?

s
FDDI is designed to handle two types of traffic:
– Synchronous frames that typically have tighter delay requirements (e.g., voice and
video)
– Asynchronous frames have greater delay tolerances (e.g., data traffic)

23. What are the features provided by layering?

Two nice features:

• It decomposes the problem of building a network into more manageable
components.
• It provides a more modular design.

24. What are 10Base2, 10Base5 and 10BaseT Ethernet LANs ?

10Base2 - An Ethernet term meaning a maximum transfer rate of 10 Megabits per second
that uses baseband signaling, with a contiguous cable segment length of 100 meters and a
maximum of 2 segments.

10Base5 - An Ethernet term meaning a maximum transfer rate of 10 Megabits per second
that uses baseband signaling, with 5 continuous segments not exceeding 100 meters per
segment.

10BaseT - An Ethernet term meaning a maximum transfer rate of 10 Megabits per second
that uses baseband signaling and twisted pair cabling. All these 3 are related to Ethernet
cables, where 10base2 can define as maximum rate of data transfer is 10megabits per
second ,B stand for baseband signaling and 2 is for 200 meter of length... this is same for
10base5 but where in 10baset T stands for twisted pair of cable

25. What is MAC address?

MAC address is abbreviated form of Media Access Control. MAC address represent in
hexadecimal format and it is 48 byte addressing scheme.eg:00-1cc0- 69-87-81.So each
quarter consists two 4 byte code and finally 48 bytes. The MAC address is stored within
the NIC(Network Interface Card).And the main purpose of MAC address is to uniquely
identify a system. The MAC address works in Layer 2(Data link layer) of ISO model.
Where layer 2 is only associated with MAC address or Physical address. And the layer 2
device switch contains MAC address table to occur switching between source and
destination. OR MAC address is a unique 48 bit serial number stored on the ROM carried
by network card adapter which is used to identify a computer in a Ethernet network.

26. What is the difference between physical address and logical address?

A Physical address is a 48-bit flat address burned into the ROM of the NIC card which is
a Layer1 device of the OSI model. This is divided into 24-bit vendor code and 24-bit
serial address. This is unique for each system and cannot be changed. A Logical address
is a 32- bit address assigned to each system in a network. This works in Layer-3 of OSI
Model. This would be generally the IP address. Simply we can say that Physical address
is MAC address (Media Access Control) which inbuilt and Logical Address is the IP
which is given manually.

27. What is difference between ARP and RARP?

The address resolution protocol (ARP) is used to associate the 32 bit IP address with the
48 bit physical address, used by a host or a router to find the physical address of another
host on its network by sending a ARP query packet that includes the IP address of the
receiver. ARP-Address resulution protocol Used to find the physical address if it knows
the logical address.

The reverse address resolution protocol (RARP) allows a host to discover its Internet
address when it knows only its physical address. RARP-Reverse address resolution
protocol Used to find the logical address if it knows the physical address.
28. What are the advantages of FDDI over a basic Token Ring?

 Fiber Distributed Data Interface (FDDI) is a LAN protocol using optical fiber as a
medium, with a 100Mbps data rate.

UNIT-II
 DATA LINK LAYER

1. What are the responsibilities of data link layer?

Specific responsibilities of data link layer include the following.

a) Framing
b) Physical addressing
c) Flow control
d) Error control
e) Access control

2. Mention the types of errors.

There are 2 types of errors

a) Single-bit error.
b) Burst-bit error.

3. Define the following terms.

Single bit error: The term single bit error means that only one bit of a given data unit
(such as byte character/data unit or packet) is changed from 1 to 0 or from 0 to 1.

Burst error: Means that 2 or more bits in the data unit have changed from 1 to 0 from
0 to 1.

4. What is redundancy?

It is the error detecting mechanism, which means a shorter group of bits or extra bits may
be appended at the destination of each unit.

5. List out the available detection methods.

There are 4 types of redundancy checks are used in data communication.

a) Vertical redundancy checks (VRC).
b) Longitudinal redundancy checks (LRC).
c) Cyclic redundancy checks (CRC).
d) Checksum.

6. Write short notes on VRC.

The most common and least expensive mechanism for error detection is the vertical
redundancy check (VRC) often called a parity check. In this technique a redundant bit
called a parity bit, is appended to every data unit so, that the total number of 0’s in the
Unit (including the parity bit) becomes even.

7. Write short notes on LRC.

In longitudinal redundancy check (LRC), a block of bits is divided into rows and a
redundant row of bits is added to the whole block.

8. Write short notes on CRC.

The third and most powerful of the redundancy checking techniques is the cyclic
redundancy checks (CRC) CRC is based on binary division. Here a sequence of
redundant bits, called the CRC remainder is appended to the end of data unit.

9. Write short notes on CRC generator.

A CRC generator uses a modulo-2 division.

In the first step, the 4-bit divisor is subtracted from the first 4 bit of the dividend.
Each bit of the divisor is subtracted from the corresponding bit of the dividend without
disturbing the next higher bit.

10. Write short notes on CRC checker

A CRC checker functions exactly like a generator. After receiving the data appended with
the CRC it does the same modulo-2 division. If the remainder is all 0’s the CRC is
dropped and the data accepted. Otherwise, the received stream of bits is discarded and the
dates are resent.

11. Give the essential properties for polynomial.

A polynomial should be selected to have at least the following properties.

a) It should not be
b) It should be divisible by(x+1).

12. Define checksum.

The error detection method used by the higher layer protocol is called checksum.
Checksum is based on the concept of redundancy.

13. What are the steps followed in checksum generator?

The sender follows these steps

a) The units are divided into k sections each of n bits.
b) All sections are added together using 2’s complement to get the sum.
c) The sum is complemented and become the checksum.
d) The checksum is sent with the data.

14. List out the steps followed is checksum checker side.

The receiver must follow these steps

a) The unit is divided into k section each of n bits.
b) All sections are added together using 1’s complement to get the sum.
c) The sum is complemented.
d) If the result is zero.

15. Write short notes on error correction.

It is the mechanism to correct the errors and it can be handled in 2 ways.

a) When an error is discovered, the receiver can have the sender retransmit the entire data
unit.
b) A receiver can use an error correcting coder, which automatically corrects certain
errors.

16. Mention the types of error correcting methods.

There are 2 error-correcting methods.

a) Single bit error correction
b) Burst error correction.

17. What is the purpose of hamming code?

A hamming code can be designed to correct burst errors of certain lengths. So the simple
strategy used by the hamming code to correct single bit errors must be redesigned to be
applicable for multiple bit correction.

18. Define flow control.

Flow control refers to a set of procedures used to restrict the amount of data. The sender
can send before waiting for acknowledgment.

19. What is a buffer?

Each receiving device has a block of memory called a buffer, reserved for storing
incoming data until they are processed.

20. Mention the categories of flow control.

There are 2 methods have been developed to control flow of data across communication
links.
a) Stop and wait- send one from at a time.
b) Sliding window- send several frames at a time.

21. Briefly discuss Stop and Wait method of flow control?

      In Stop and Wait of flow control, the sender sends one frame and waits for an
acknowledgement before sending the next frame.
 22. In the Hamming code for a data unit of m bits how do you compute the
number of redundant bits ‘r’ needed?
In the Hamming code, for a data unit of m bits, use the formula 2r > = m + r + 1 to
determine r, the number of redundant bits needed.

23. How does ARQ correct an error?

Anytime an error is detected in an exchange, a negative acknowledgment (NAK) is
returned and the specified frames are retransmitted.

24. What is the purpose of the timer at the sender site in systems using ARQ?
The sender starts a timer when it sends a frame. If an acknowledgment is not received
within an allotted time period, the sender assumes that the frame was lost or damaged and
resends it.

25. What is damaged frame?

A damaged frame is recognizable frame that does arrive, but some of the bits are in error
(have been altered during transmission)

26. What is HDLC?

HDLC is a bit oriented datalink protocol designed to support both half-duplex and full
duplex communication over point to point and multiport link.

27. Give data transfer modes of HDLC?

1.   NRM – Normal Response Mode
2.   ARM – Asynchronous Response Mode
3.   ABM - Asynchronous Balanced Mode

28. How many types of frames HDLC uses?

1.   U-Frames
2.   I-Frames
3.   S-Frame

29. State phases involved in the operation of HDLC?

1.   Initialization
2.   Data transfer
3.   Disconnect

30. Define piggybacking?

The inclusion of an acknowledgment to a previously received packet in an outgoing data
packet is known as piggybacking.

31. What is the meaning of ACK frame?

ACK frame is an indication that a station has received something from another.

32. Write the design issues of datalink layer?

1)   Services provided to network layer.
2)   Framing
3)   Error control
4)   Flow control

33. What is datalink?

When a datalink control protocol is used the transmission medium between systems is
referred to as a datalink.

34. What is the main function of datalink layer?

The datalink layer transforms the physical layer, a raw transmission facility to a reliable
link and is responsible for node to node delivery.

35. What is a datalink protocol?

Datalink protocol is a layer of control present in each communicating device that
provides functions such as flow control, error detection and error control.

36. What is meant by flow control?

Flow control is a set of procedures used to restrict the amount of data that the sender can
send before waiting for an acknowledgement.

37. How is error controlled in datalink controlled protocol?

In a datalink control protocol, error control is activated by retransmission of damaged
frame that have not been acknowledged by other side which requests a retransmission.

38. Discuss the concept of redundancy in error detection.

Error detection uses the concept of redundancy, which means adding extra bits for
detecting errors at the destination.

39. How can the parity bit detect a damaged data unit?
In parity check, (a redundant bit) a parity bit is added to every data unit so that the total
number of 1s is even for even parity checking function (or odd for odd parity).

40. How can we use the Hamming code to correct a burst error?
By rearranging the order of bit transmission of the data units, the Hamming code can
correct burst errors.
41. What is Byte Stuffing?

 Byte stuffing is the process of adding 1 extra byte whenever there is a flag or escape
character in the text.

42. What are the three protocols used for noisy channels?

Stop – and – Wait ARQ ,Go – back – N  ARQ, Selective Repeat  ARQ

43. Define Error Control & Flow Control.

 Error Control: It is based on automatic repeat request, which is the retransmission of
data.

 Flow Control: It refers to a set of procedures used to restrict the amount of data that the
sender can send before waiting for acknowledgment.

44. Define Bit Stuffing.

Bit stuffing is the process of adding one extra 0 whenever five consecutive 1s follow a 0
in the data, so that the receiver does not mistake the pattern 01111110 for a flag.

45. Why do you need Bit Stuffing in HDLC?

 Bit stuffing is the process of adding one extra 0 whenever there are five consecutive 1s
in the data, sothat the receiver does not mistake the data for a flag.

46. What is hamming distance

The number of corresponding bits that differ between two codewords is the Hamming
distance of those two codewords. For example, the Hamming distance between the
codewords 1001 and 0101 is 2. The Hamming distance of two codewords can be
calculated as the number of 1 bits in the bitwise exclusive-or of the two codewords: 1001
xor 0101 = 1100.

47. What is minimum hamming distance

A code is the set of all codewords of a given length that are constructed by adding a
specified number of check digits in a specified way to a specified number of data bits.
The minimum Hamming distance of a code is the minimum of the Hamming distance
between all possible pairs of codewords of that code.

48. Compare and contrast Go back N and selective Repeat

Go-Back-N ARQ

 Retransmission begins with the last unacknowledged frame even if subsequent

frames have arrived correctly .duplicate frames are discarded.

 Go-back-n ARQ -- Receiver must get Frames in correct order

Selective Repeat ARQ

  Only the unacknowledged frame is retransmitted.

 It may be (slightly) more efficient than Go-back-n ARQ, but also much more
complicated.

 Selective repeat ARQ -- correctly-received out-of-order Frames are stored at

Receiver until they can be re-assembled into correct order
49. Define error control and flow control

flow control specifies how much data the Sender can transmit before receiving
permission to continue from the Receiver

error control allows the Receiver to tell the Sender about frames damaged or lost during
transmission, and coordinates the re-transmission of those frames by the Sender

50. What is the window size for Go back N and Selective repeat

Go Back N : < 2m

Selective Repeat : 2m - 1